Historically, the design of intake manifolds for internal combustion engines has involved the sacrifice of torque output and performance at different engine speeds. This is because optimum air flow patterns and volumetric requirements of an engine at one engine speed and load, differ considerably from optimum conditions at other engine speeds and loads. It has proven difficult to obtain one manifold design suitable for a wide range of engine speed requirements.
Air intake manifolds generally comprise a manifold body formed with a plenum having an inlet connected to an air or throttle valve. A number of air passages or runners are formed in the manifold body having an inlet at the plenum interior and an outlet connected to one of the cylinders of the engine. A flow of air is directed into the plenum interior and then distributed into the several runners for transmission to the cylinders of the engine where it is intermixed with fuel supplied by fuel injectors. Air-fuel intake manifolds are generally similar in construction except the plenum inlet is connected to a fuel injection or carburation system which discharges a mixture of fuel and air into the plenum for distribution to the runners.
In an engine intended to operate predominantly at high speeds, air and air-fuel intake manifolds have generally been designed with runners having a relatively large cross section. This is because at high operational speeds the engine requires a substantial quantity of air and fuel to support combustion, and thus a large cross sectional area must be provided within each runner to produce sufficiently high flow rates of air or air-fuel therethrough. While this design provides acceptable efficiency and torque at high engine speeds, the performance of the engine while idling or at lower "street" speeds with manifolds of this type is extremely poor. The large area runners make it virtually impossible to obtain the desired air velocity and air volume at lower speeds, and therefore torque output is reduced, with an accompanying drop in fuel economy and an increase in hydrocarbon emissions.
Essentially the reverse problem occurs with intake manifolds designed for engines to be operated primarily at lower speeds. In these manifold designs, the runners of the manifold are typically formed with relatively small cross-sectional areas so that higher velocity flows of air and/or an air-fuel mixture can be obtained at lower engine speeds. But because of the comparatively small size of the runners in such manifolds, insufficient flows of air and/or an air-fuel mixture are provided at high engine speeds thus severely limiting the engine performance and torque output. Consequently, the design problem has been one of obtaining the desired engine performance and torque output through more than a narrow range of engine speeds.